Render gpu use 1 core cpu!
When rendering an image using the GPU, the data somehow needs to be loaded from your disk, preprocessed, pushed into RAM, from there into VRAM where the GPU finally starts doing its job. Everything up until the point where the data enters VRAM is done by the CPU. 

If your GPU is working at 100% there is simply no need to push more data into this pipeline, so it's actually a good idea to not occupy more of the CPU's cores than necessary. In fact, this one CPU core won't even reach 100% unless your system runs out of RAM. In this case the CPU starts swapping the content of the memory onto the hdd, which ultimately will rise the CPU load. 

So there's no solution needed, the system manages its own resources quite fine.